Root Access Points (APs) are connected to the network infrastructure via an Ethernet link. Non-root “child” APs are connected to a root or non-root “parent” AP over an 802.11“backhaul” radio link. A parent AP uses a “downlink” radio and corresponding downlink antenna to communicate with child radios; a child AP uses an “uplink” radio and corresponding uplink antenna to communicate with a parent radio in a parent AP. A non-root AP may use a single “uplink/downlink” radio to both communicate with its parent and with its children.
An “antenna scheduling problem” is introduced if parent and child mesh radios are connected via switched sector antenna arrays. The parent and child radios must agree on “rendezvous times,” when both the parent and child radio transceivers are connected to the pair of sector antennas that provide the parent/child link. A fixed-time-slot protocol can be used to resolve the antenna scheduling problem. A parent radio establishes a schedule in which it visits each sector antenna in the array for a fixed time slot. A fixed-time-slot protocol, however, introduces an “idle slot problem,” in which time slots are completely or partially unused. For example, consider a transient condition in which a station is sending a large file (e.g. over a single backhaul antenna) and all other stations are temporarily idle. In such a scenario, a fixed-time-slot protocol effectively reduces the available bandwidth by a factor equal to the number of antennas. For example, if 8 fixed time slots are allocated to 8 antennas, then only ⅛th of the bandwidth can be used for the file transfer. Moreover, it is difficult to use an entire fixed time slot, because a radio cannot initiate a frame transmission if it cannot be completed within the fixed time slot (i.e. without losing the frame). The problem can be partially resolved by “fragmenting” transmissions; however, fragmentation adds significant additional overhead, e.g. for the per-fragment preamble in physical layer/media access control layer (PHY/MAC) headers and per-fragment Frame Check Sequence (FCS) values. A “variable-time-slot protocol” can be used to partially resolve the “idle slot problem.” Instead of scheduling fixed time slots per antenna, a parent radio can schedule variable time slots that are adapted to the offered load on each antenna. In order to maintain a variable schedule, the variable-time-slot protocol is relatively complex and “chatty.” A parent radio must constantly send frames on both active and idle antennas so that child radios can maintain synchronization with the variable antenna schedule. In addition, bandwidth is wasted if a parent radio and child radios cannot agree on accurate rendezvous times.
A co-located radio coordination problem is introduced if a first radio and a second radio are “co-located.” Herein, two radios are “co-located” if the radios are contained in the same device (e.g. 802.11 access point) and operate on radio frequencies that are not widely separated. If a first radio transmits at the same time that a second co-located radio is receiving a frame, the frame reception on the second radio will be corrupted. If a first radio transmits at the same time that a second co-located radio is transmitting a frame, the frame transmissions may or may not be corrupted at the respective receivers, depending upon conditions such as the spatial separation of the receivers and the transmit antenna patterns.